Modern access systems support different higher layer protocols. Protocols define the format and order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. The central purpose of the Medium Access Control (MAC) protocol is sharing of radio channel resources. The MAC protocol defines how and when an access point or subscriber unit may transmit on the channel. The MAC protocol includes the interface's procedures to provide guaranteed services to upper layers.
Wireless medium is a shared medium, which demands the MAC protocol to co-ordinate the transmission of multiple traffic flows over it. The basic distinction between different MAC protocols is the duplexing of the uplink and downlink channels. In Time Division Duplex (TDD), the downlink and uplink channels use the same carrier frequency. The data unit, i.e. MAC frame, is divided into an uplink portion and a downlink portion. The border between the uplink and downlink portion can be adaptive, which makes it suitable for asymmetric connections. In Frequency Division Duplex (FDD), different carrier frequencies are used in the downlink and uplink transmission. The terminals may thus simultaneously transmit and receive the signals. Finally, in Half-duplex Frequency Division Duplex (H-FDD), different carrier frequencies are used for the uplink and downlink transmission, but the terminals do not transmit and receive simultaneously. This poses a challenging problem to the uplink and downlink resource management. Furthermore, the type of physical channel has a significant influence on the radio access protocol and scheduling procedures. In a continuous transmission channel, the traffic flow is transmitted in the downlink direction and the whole traffic flow is received in the access point of the access network. The terminals have to decode the whole flow and pick up the packets addressed to them. In a Time Division Multiplexing (TDM) stream channel, the modulation type is changed within one MAC frame. The change has to be announced at the beginning of the MAC frame. The packets intended for various terminals have to be re-ordered according to the modulation type used by a particular terminal. In a Time Division Multiple Access (TDMA) burst channel, a standby mode is allowed when the data is not addressed to a particular terminal. The frame structure is announced at the beginning of the MAC frame.
An example of a wireless communication system, where FDD and a burst mode of transmission are adopted, and support of half-duplex terminal is required, is the air interface for the IEEE 802.16 fixed broadband wireless access system. In this burst-mode FDD system, the downlink channel is framed to allow adaptive modulation and forward error correction (FEC). To accommodate half-duplex terminals, the downlink channel uses TDMA or a mixture of TDM and TDMA, where TDM is utilized for bandwidth efficiency and TDMA is used for half-duplex terminal support. Furthermore, downlink and uplink burst transmissions are centrally scheduled on a frame-by-frame basis by a central controller or access point (AP), in order to meet specified quality of service (QoS) requirements. Scheduling deals with the manner in which queued data packets are selected for transmission on the respective link or channel. A downlink map message, transmitted at the beginning of each frame, broadcasts the frame layout to all other terminals in the system.
However, the QoS requirements can impose very tight constraints on the AP scheduler, which has to determine which packets to transmit next, and when, in order to meet system-defined QoS requirements. Similarly, half-duplex terminal transmission and reception scheduling imposes additional tight constraints which are independent of the QoS requirements. In particular, the burst data transmission order in each frame has to be arranged in such a manner that, for each of the half-duplex terminals, transmission and reception intervals do not overlap in time.